A long evolutionary history of biosonar-guided aerial foraging has refined bats' auditory processing capabilities tremendously. Bats are, therefore, excellent subjects for studying basic auditory functions in mammals. In some ways, such as the close coordination of sound production and hearing and the sophisticated processing applied to sounds, bats' sonar can even be likened to human language. The general goal of this proposal is to increase knowledge of mammalian auditory processing through study of bat biosonar. How bats analyze sonar signals will be investigated behaviorally using electronically simulated echoes to create virtual ("phantom") targets. The characteristics of different bats' sonar sounds appear to be unique -- individuals have their own stereotyped "personal" emissions. This makes it possible to characterize a bat's emissions mathematically and make models mimicking the emissions. These models can be used as simulated echoes to produce phantom targets. The characteristics of model echoes can easily be manipulate to create virtual targets of different sonar "appearance," and, by training bats to discriminate among two or more targets of different appearance, bats' auditory processing capabilities can be studied. In the experiments proposed here, model echoes will be delivered through headphones, which allows excellent control of the sound stimuli reaching each ear. This will permit investigation of the echo features most salient for target identification (e.g., intensity, spectral, or range-profile variation) and of the cues used to determine the horizontal angle to a target (interaural intensity differences or echo time-of-arrival differences). In addition, bats' sensitivity to differences in the phase of the ultrasonic frequency components of echoes will be studied in order to better characterize the bat's sonar receiver. Taken together, the results of these experiments should give us a much better idea of how bat biosonar operates and of mammalian signal processing strategies in general.